The present invention relates generally to power supplies utilizing transformer isolation and requiring very fast isolated output voltage turn-off, such as may be useful in grid-controlled x-ray tubes, for example.
U.S. Pat. No. 5,969,955 describes a power supply utilizing an isolation transformer including a crowbar circuit and control such that turn-off by discharging an output capacitor is made advantageously faster than conventional circuits. The forward converter has a push-pull arrangement of two switching devices. For operation in the output voltage-on mode, the forward converter transfers energy in a peak forward converter mode with the first switching device being turned on and off. For output voltage turn-off, the crowbar circuit is activated by turning on the second switching device, resulting in a reverse voltage at the secondary winding of the isolation transformer. The reverse voltage is superimposed on the converter output voltage, the sum of the converter output voltage and the reverse voltage being imposed across the crowbar circuit. If the sum of the converter output voltage and the reverse voltage exceeds the breakover voltage of the crowbar circuit, the output capacitor discharges rapidly through the crowbar circuit and the transformer secondary winding.
It is desirable to provide a push-pull circuit arrangement and control that minimizes the number of components in the crowbar circuit, hence providing a more compact and lower cost circuit design. It is furthermore desirable to provide such a circuit and control that maximizes efficiency and reliability.